A few yrs ago I wanted to use my serger thread to do some FSL projects on my emb machine so that it would be cheaper and since it was for kids didn't want to spend a fortune on thread knowing the project would be around long. At first it would do good, then break there was no ryhme or reason as to when it would do this. So I thought ok, the thread is close to 18yrs old I should get new thread, I did and I got the same results. Took me a while but finally figured out to change the needle to a larger size and slow it down. I'm now back to doing serger projects and use old and new and it never breaks on me. If my thread were constantly breaking then I'd probably worry but it takes a lot of stress on the thread going thru a serger so if it doesn't break in the machine I wouldn't worry about it. Like I said, most of mine is around 18yrs old.
I had recently bought some from Joann's and it started breaking so I took it back and they had no problems refunding my money.